Transient recovery voltage is the transient across a circuit breaker. TRV is a decisive parameter that limits the interrupting capability of a circuit breaker.TRV manifests in different ways depending on circuit configuration, hence the objective of this report is to study the effects caused by the TRV and to validate the suitability of the proposed circuit breakers or other alternatives to reduce the effects of TRV in case it exceeds tolerable limits in the LNG plant under study. LNG Plants involve electrical system with many trains and may need a longer transmission of power between trains. They also have more reactive loads. Hence, effect of TRVs on the CBs of such system is critical in making the choice of CBs.
The TRV ratings define a withstand boundary. A circuit TRV that exceeds this boundary is in excess of the circuit breakers rated or related capability. However, the study shows that the highest TRV obtained is 19.3kV and is within limits of the Circuit Breaker TRV capabilities (23.5 kV) as per IEC IEC62271-100.
As the withstand boundary, including the time to peak and RRRV are within limits with adequate margin, the proposed selection of the 11kV Circuit Breaker is acceptable for use in the LNG plant.
Recommendations:
The power system of this LNG plant need not be proposed with methods to reduce the effect of TRV. The reasons for TRV being within the limits are that the loads are concentrated in a small geographical area and are not capacities. However, system involving long transmission lines, high capacities loads, plants at direct-stroke lightning prone areas, cases involving transformer inrush during energisation etc. may have intolerable TRVs in the system and may need suitable remedy.